On an aircraft every pound of additional weight translates directly into increased fuel burn and increased operating costs for the airlines. As airlines tend to purchase and operate only the safest, most cost and fuel efficient aircraft available on the market, obvious financial penalties exist for systems which are delivered overweight. Of paramount importance to an aircraft system designer then, in addition to the obvious safety and reliability concerns, is system weight. Since many of the design parameters are fixed, the designer must utilize advanced techniques to reduce overall system weight.
One of these advanced techniques utilized for electric power generation systems on aircraft is the use of parallel power feeders for power distribution from the engine mounted generators to the electrical power distribution center. With this technique two small aluminum power feeders are used to conduct the current generated by each electrical phase of the generator, as opposed to a single larger copper feeder for each phase as is typical in ground based power distribution systems.
FIG. 1 illustrates a partial single channel of an aircraft electric power generation system utilizing this advanced technique. Each output electrical phase 10, 12, 14 of the engine driven generator 16 is coupled by a single copper power feeder 18, 20, 22 to a terminal block 24 within the aircraft outside the engine nacelle (not shown). These copper power feeders 18, 20, 22 are required due to the harsh environment of this nacelle area through which they are routed. Once within the body of the aircraft however, two parallel aluminum feeders 18a and 18b, 20a and 20b, 22a and 22b are utilized to conduct the electrical energy produced by the generator 16. These parallel feeders 18a and 18b, 20a and 20b, 22a and 22b are coupled to each phase on the terminal block 24, grouped into two three-phase bundles, and routed through the aircraft to a second terminal block 26 which is located near the electrical power distribution center in the nose of the aircraft (not shown). From this terminal block 26 a single copper power feeder 18c, 20c, 22c conducts the electrical energy through the generator line contactor 28 to the load distribution buses (not shown).
To achieve a significant weight savings, each aluminum parallel feeder 18a, 18b, 20a, 20b, 22a, 22b is sized to conduct only one half of the rated capacity per phase of the generator. A problem associated with this sizing decision, however, is that if one of these small parallel feeders 18a were to break or become disconnected from the terminal block 24, the other parallel feeder 18b would be required to conduct the full capacity of that phase of the generator. Under heavy loading conditions, the unfaulted parallel feeder 18b will become overloaded and may present a fire hazard.
To prevent such an occurrence, two, single torroid parallel feeder current transformers 30, 32 are used to sense the current conducted by each parallel feeder bundle. The output of one parallel feeder current transformer 30 is cross coupled to the output of the other parallel feeder current transformer 32. FIG. 2 illustrates the sensed current vector subtraction which results from this cross coupling of outputs. In FIG. 2a the distribution system is free of faults; the current carried in each parallel feeder bundle is equal; and the output transmitted to the level detector circuitry 38 of the generator control unit 34 via line 36 is zero.
If one of the parallel feeders were to open however, as illustrated in FIG. 2b, the current sensed by each current transformer will no longer be equal and the vector subtraction will no longer result in a null output. This non-zero result is transmitted to the level detector circuitry 38 which then generates a fault flag on line 40. Protection logic 42 within the generator control unit 34 processes this flag and generates a protective trip signal on line 44. The exciter control circuitry 46 is responsive to this trip signal to de-energize the generator 16 by removing the excitation drive current from line 48. The generator line contactor driver circuitry 62 is also responsive to the protective trip signal to trip open the generator line contactor 28 via line 63 to disconnect the generator 16 from the load buses.
FIG. 2c illustrates the need to cross one of the parallel feeders 18a, 18b from each parallel feeder bundle to the other just prior to the parallel feeder current sensors 30, 32. For this vector diagram it is assumed that all parallel feeders remain in their respective bundle from the first terminal block 24 to the second terminal block 26, that is no parallel feeder is crossed over as shown in FIG. 1, and that one of the parallel feeder bundles is completely severed in half. A comparison of the resultant of the vector subtraction with the resultant of the no-fault condition as illustrated in FIG. 2a reveals that, although half of the parallel feeders are conducting two times their rated capacity, no fault is indicated to the generator control unit 34. As illustrated in FIG. 2d, by transposing one of the parallel feeders just prior to the parallel feeder current sensors (FIG. 1), the same fault severing completely one of the parallel feeder bundles is now detectable by the generator control unit 34.
One problem associated with this parallel feeder transposition, however, is that accurate current level sensing is not possible at low loading levels when one of the transposed parallel feeders is faulted. Errors as great as 33 percent have been observed in some systems utilizing this parallel feeder bundle sensing technique. Although it is not entirely clear why this error is induced into the system only when the transposed feeder is faulted, it is believed that the induced impedance of both bundles is affected by the transposition at the point of sensing.
Another problem associated with this parallel feeder transposition relates to manufacturing time and error. The additional time required to un-bundle the parallel feeders and route them through the current sensors adds an additional burden on the manufacturing process which increases costs. The potential for manufacturing error also is increased for this system as the six parallel feeders are required to be routed through two current sensors after two of the feeders are transposed. Problems of the wrong parallel feeders in the wrong current sensor are typically found during aircraft testing where the cost of rework to both schedule and profit is very high.
Still another problem associated with the prior art systems utilizing parallel feeder bundle sensing is that, in addition to the generator current sensors 50, 52, 54, separate line current sensors 56, 58, 60 are required to detect other distribution faults such as single and multiple phase to phase and phase to neutral low impedance faults. These additional sensors increase manufacturing and design costs, decrease control unit reliability, as well as increasing the overall complexity of the system.
The present invention is directed to overcoming one or more of the above problems.